1. Field of the Invention
The present invention relates to an ion implantation method, and more particularly to an ion implantation method capable of preventing a channeling phenomenon caused by a lattice structure of a semiconductor substrate.
2. Description of the Prior Art
Recently, when fabricating semiconductor devices, an ion implantation method is widely used for forming impurity areas in a substrate, such as a deep well, a source region and a drain region of a field effect transistor, a channel area, and an anti-punch through area. This is because the ion implantation method can form an impurity area having a desired doping profile while finely adjusting the impurity area as compared with a diffusion method.
FIG. 1 is a view for explaining a conventional ion implantation method without a tilt angle. As shown in FIG. 1, after forming a photoresist pattern 12 on an upper surface of a semiconductor substrate 10, an ion implantation is carried out without the tilt angle by using the photoresist pattern 12 as a mask such that a predetermined impurity layer is formed, that is, the ion implantation is carried out in a state in which the tilt angle is zero.
Generally, the semiconductor substrate 10 is made from single crystalline silicon material. In this case, the semiconductor substrate 10 has a lattice structure in which silicon atoms 10a are uniformly aligned.
However, according to the convention ion implantation method without the tilt angle, an implanted ion 10b may not make contact with silicon atoms 10a of the lattice structure, but pass through gaps formed between silicon atoms 10a, called a “channeling phenomenon”, so it is difficult to form an impurity layer having a desired profile.
To solve the above problem caused by the channeling phenomenon, an ion implantation method utilizing a tilt angle and a twist angle when implanting ions has been proposed.
FIG. 2 is a view showing a conventional ion implantation method adopting a tilt angle when implanting ions. Referring to FIG. 2, after forming a photoresist pattern 22 on an upper surface of a semiconductor substrate 20, an ion implantation is carried out with a predetermined tilt angle by using the photoresist pattern 22 as a mask such that a predetermined impurity layer is formed on the semiconductor substrate 20.
Generally, the semiconductor substrate 20 is made from single crystalline silicon material. In this case, the semiconductor substrate 20 has a lattice structure in which silicon atoms 20a are uniformly aligned.
According to the conventional ion implantation method adopting the tilt angle, implanted ions 20b may frequently make contact with silicon atoms 20a, thereby preventing the channeling phenomenon. However, due to a step difference between an ion implantation angle and the photoresist pattern 22 of the semiconductor substrate 20, ions may not be implanted into a predetermined area of the semiconductor substrate 20, which is called a “shadowing effect”. In addition, ions scattered from the photoresist pattern 22 is implanted into a predetermined area of the semiconductor substrate 20, thereby distorting a doping profile of an impurity area, which is called a “scattering effect”.
Recently, as a size of a semiconductor device becomes reduced, the shadowing effect and the scattering effect may cause an asymmetric doping profile of the impurity area. As a result, electrical characteristics of peripheral circuits, such as a sense amp, are deteriorated.